1. Field of the Invention
The present invention relates to a projection lens suitable for use on a projector for projecting an image formed on a film or a slide or an image displayed by a liquid crystal display in an enlarged image on a screen, and a projector provided with the same projection lens.
2. Description of the Related Art
A projector lens for a low-cost LCD projector is required to have a large numerical aperture, to be compact and lightweight, to be capable of forming a high-definition image, to be capable of preventing distortion, to be capable of preventing chromatic aberration of magnification, to be telecentric, to have a long back focal distance, to have a short projection distance and to be inexpensive.
The projection optical system of a projector for projecting an image formed on a film or a slide or an image displayed by a liquid crystal display in an enlarged image on a screen often uses a telecentric zoom lens. However, the zoom lens is expensive and increases the cost of the projector.
A high-definition image can be formed, distortion can be reduced and chromatic aberration of magnification can be prevented by using a plurality of lenses. However, the transmittance of the projection system with light rays of short wavelengths is reduced due to internal absorption when the number of the lenses is increased. The internal absorption of light rays of short wavelength by the lenses can be reduced by forming the lenses of lead glass, which, however, increases the cost of the projection optical system.
Accordingly, it is an object of the present invention to solve problems in the prior art and to provide a projection lens consisting of a small number of lenses, having simple construction and capable of satisfactorily reducing various kinds of aberrations, and a projector provided with the same projection lens.
To achieve the object, the present invention provides a projection lens including a first lens having a negative refractive power; a second lens having a positive refractive power; a third lens having a negative refractive power and consisting of a first component lens having a negative refractive power and a second component lens having a positive refractive power; and a fourth lens having a positive refractive power; wherein the first, the second, the third and the fourth lens are arranged in that order from a screen side toward an image surface side to form a telecentric system toward the image surface side, and a surface on the image surface side of the first lens, and a surface on the image surface side of the second component lens of the third lens are aspherical.
Since the surface on the image surface side of the first lens, and the surface on the image surface side of the second component lens are aspherical surfaces, aberrations including distortion, coma and astigmatism can be reduced even though the projection lens consists of the small number of lenses.
In the projection lens according to the present invention, 0.7 less than |f1/f2-4| less than 1.0, where f1 is the focal length of the first lens, and f2-4 is the composite focal length of the second lens, the third lens and the fourth lens, and the focal length f1 and the composite focal length f2-4 have opposite signs, respectively.
Thus, the first lens is able to gather light rays from a wide angular range, and the back focal distance is long relative to the focal length f of the projection lens.
In the projection lens according to the present invention, 0.8 less than f2/f4 less than 1.2, where f2 is the focal length of the second lens, and f4 is the focal length of the fourth lens, the radius of curvature of a surface on the screen side of the second lens is smaller than that of the other surface on the image surface side of the second lens, and the radius of curvature of a surface on the screen side of the fourth lens is greater than that of the other surface on the image surface side of the fourth lens.
Thus, aberrations caused by the second and the fourth lens compensate each other and hence a distortion that may be caused by the second and the fourth lens can be reduced.
In the projection lens according to the present invention, the aspherical surface on the image surface side of the first lens is expressed by:   x  =                    Ry        2                    1        +                              1            -                                          (                                  1                  +                  K                                )                            ⁢                              R                2                            ⁢                              y                2                                                          +          Ay      4        +          By      6        +          Cy      8        +          Dy      10      
where A, B, C and D are predetermined fixed values, and R meets 0.5 less than R/f less than 1.0, where f is the focal length of the projection lens.
Thus, the surface on the image surface side of the first lens can be formed in a shape that does not cause large aberrations including a distortion, a coma and an astigmatism.
In the projection lens according to the present invention, a surface on the image surface side of the first lens is formed in an aspherical shape by coating a spherical surface of the glass body of the first lens with a thin film of a UV-curable resin, and finishing the outer surface of the thin film in an aspherical shape.
Thus, the surface on the image surface side of the first lens can be easily formed in the aspherical shape.
A projector according to the present invention includes an image forming means for forming an image, and the projection lens according to the present invention for projecting an image formed by the image forming means.
Thus, the projector is capable of projecting an image in a high image quality.